In recent years the use of magnetized paddle wheels for measuring the speed of a vessel have been used extensively. Paddle wheels being rotatably mounted on the side or bottom of the hull such that the tips of the paddles having opposite charges rotate very closely past a "hall-effect" device, are used extensively.
Such a device is shown in U.S. Pat. No. 4,644,788 wherein the paddle wheel is incorporated into a separate paddle wheel assembly. Struts hold the paddle wheel in a rotatably mounted position. The blades of the paddle wheel extend downwardly, out of the housing, in order to be in contact with the water flowing along the bottom of the hull of a boat when the boat is in motion. In U.S. Pat. No. 4,644,788 the housing on which the paddle wheel is mounted is removable from the other components for the purpose of cleaning off debris or weeds which collect on the paddle wheel and restrict its rotational speed.
In U.S. Pat. Nos. 4,898,029, and 4,836,020, entitled Marine Instrument, a sensing device which consists of a paddle wheel having a plurality of magnetized paddles extending from a central hub is disclosed. The paddle wheel is mounted within a cavity formed within the lower portion of a housing. The housing passes through the hull of the boat a certain distance in order to avoid a "boundary effect". The ends of the paddles, during rotation, extend below the housing in order to be moved by the flow of water.
All of the prior art known devices suffer from one particular disadvantage in that the arrangement of the paddle wheel is such that the paddles extend outwardly, beyond the surface of the hull, whether it be the side, bottom or other portion thereof. Consequently, such devices are open to damage from rocks or obstacles. In weedy lakes the paddles collect debris.
The protection of the speed measuring device and the possibility of damage is not severe in larger marine vessels which always remain in deep water. However, the problem is quite severe for smaller watercraft which are sometimes pulled or driven up on the shore.
The damage problem is particularly severe when known speed measuring devices are used with the new personal watercraft such as those sold under the trade-marks SEA-DOO* and JET-SKI*. This type of watercraft is often driven up onto sandbars as they are propellerless, being powered by jet pumps. Thus, the known prior art speed measuring devices are not suitable for this line of watercraft because the paddles of the paddle wheel extend beyond the bottom of the hull of the craft in an unprotected position.
It is therefore an object of the present invention to provide a speed measuring device for use with personal jet pump propelled watercraft wherein the moving parts of the device do not protrude beyond the boundaries of the hull.
It is a further object of the present invention to provide a speed measuring device wherein a rotatable paddle wheel is protected within a cavity formed by a housing and is easily adaptable to installation on a personal watercraft.